This invention is in the field of semiconductor devices insensitive to hostile radiation and more specifically is related to field effect devices whose gate electrodes consist of materials whose work function can be changed.
The MOSFET (metal-oxide-semiconductor-field-effect-transistor) device is a generic name for a broad class of solid state devices such as PMOS, NMOS and CMOS. The operation of MOSFETs and similar devices such as insulated gate transistors depends upon the creation of an inversion charge layer between the "source" and "drain" electrodes by application of a voltage to the "gate" electrode. Upon application of this voltage, conduction between source and drain commences. This voltage is called the threshold voltage and is given theoretically by the equation: EQU V.sub.T = .phi. - (.sigma./.epsilon.) d - F.
Where .phi. is the work function of the metal electrode, .sigma. is the total charge at the oxide-semiconductor interface, .epsilon. is the oxide dielectric constant, d is the oxide thickness, and F a term which is relatively constant. If the conduction carriers between source and drain are electrons (holes), the MOSFET is called an N-channel (P-channel) device. For an N-channel and P-channel device the threshold voltages, relative to each other, are positive and negative, respectively. However, if somehow, positive interface charge, .sigma., were added then V.sub.T would decrease for both N- and P-MOS devices. For the former the device would be easier to "turn-on", whereas for the latter the opposite would be true. The situation can occur where there is enough .sigma. to make an N-channel device turn on at V.sub.T = O. This is, indeed, what happens to these devices in a hostile radiation environment. The radiation causes plasmons to be excited in the oxide. These, in turn, decay into electronhole pairs; the holes under the appropriate gate voltage are transported to the oxide-semiconductor interface and subsequently trapped by presently unknown means. The end result is an increase in .sigma., decrease in V.sub.T, and a vital change in circuit parameters which can be catastropic for a sensitive electronic package. Additionally, although some annealing occurs with time and elevated temperature, the increase in .sigma. is for many practical applications irreversible.
Various techniques have been utilized to render similar devices immune to unfriendly radiation. M. J. Rand and P. F. Schmidt disclose in U.S. Pat. No. 3,795,935 a silicon oxynitride dielectric coating which prevents both the formation of space charge in the dielectric and the formation of interface states. Although providing a high degree of insensitivity, it does not render the device totally immune to radiation effects. U.S. Pat. No. 3,882,530 to Danchenko discloses radiation hardening of a MOSFET device by introducing boron into the oxide insulating layer of the device. The technique disclosed utilizes a self-recovery mechanism for which a period of time must lapse before it returns to its pre-radiation electrical state.
The present invention utilizes the properties exhibited by the transition metals and alloys thereof of absorbing hydrogen interstitially. This absorption in these materials is reversible. See K. I. Lundstrom, M. S. Shivaraman, and C. M. Svenson, J. Appl. Phys, 46,3876 (1975). One change exhibited in this process is in the metal's work function. What this inventor has discovered is that by utilizing this property with appropriate circuitry and means for controlling the amount of hydrogen present in the electrode the device can be rendered insensitive to radiation environments. Additionally, the device can be made responsive to other influences affecting the gate electrode capacitance -- voltage characteristics.
It is therefore one object of this invention to provide a solid state device which is insensitive to the effects of a harmful radiation environment.
Another object of this invention is to provide a device which retains its vital electrical parameters rapidly upon subjection to a hostile radiation environment.
A further object of this invention is to provide a solid state device whose vital parameters are unaffected by it being made a radiation hardened device.
Still another object of this invention is to provide a gate field effect transistor, utilizing special electrodes and their related hydrogen properties, so its work function can be altered to compensate for changes in the threshold voltage of the device.